The present invention pertains to the hydraulic valve art and, more particularly, to an improved integral blocking and relief valve.
Numerous hydraulic blocking valves have been developed in the prior art. A common application for such valves is in the aircraft flight power control actuator art wherein, for example, blocking valves are used in spoiler actuator systems. During flight, it is crucial to the safe operation of the aircraft that the spoiler be in its retracted position when not commanded otherwise. Thus, to prevent spoiler surface motion as a result of reduced system pressure or external loading, it has been common practice to employ a blocking valve which traps hydraulic fluid in the actuator thereby locking the spoiler in place.
The pressure of this trapped hydraulic fluid can increase due to thermal effects or external loading. To prevent an excessively high pressure build up, the prior art has utilized a thermal relief valve which bleeds fluid to system return until the pressure is relieved.
Attempts have been made in the prior art to combine both the blocking and relief valve functions in a single valve unit, thereby saving in weight, construction cost and space. A problem with one such design is that pressure relief is biased upwards by system pressure. That is, in this prior design the pressure relief occurs at nearly twice the pressure with system pressure on than it does when system pressure is off. A weight penalty is incurred in such actuator designs to accommodate the higher than desired working pressures.
In another previous integral blocking and relief valve design, the system is successful in providing a constant relief setting regardless of system pressure. However, this design utilizes a large number of parts thus rendering it expensive to manufacture and potentially less reliable in operation.